1. Field of the Invention
The present invention relates generally to gear trains having anti-backlash devices and, more particularly, to an index table apparatus having a gear drive assembly for rotating the table to a desired angular position, wherein the gear drive assembly includes a duality of driving members having between them a phase shift apparatus for removing backlash within the gear train.
2. Discussion of the Prior Art
It is known to provide an index table for use in supporting a workpiece and for rotating the workpiece to various preset angular positions at which assorted operations are carried out. For example, indexing tables are used in the manufacture of machine tools, and are operable to reposition a tool during machining operations so that a number of such operations may be conducted without removing the tool from the table. In such indexing tables, rotary motion is provided either manually, or automatically as in the case of a numerically controlled device.
Where rotary motion of an indexing table is carried out automatically, it is conventional to provide a motor and a gear drive assembly for transmitting rotational movement of the motor to the table in a manner capable of being measured and controlled. For example, it is possible to transmit rotational movement of a DC servomotor to an indexing table via a gear train and to stop and lock the indexing table at a preset angular position by actuating an air brake or the like provided on the device when the desired position is reached.
A number of drawbacks are present in a device of this known type. For example, although air brakes and the like reliably stop an indexing gear and retain the gear in a locked position, such devices fail to accurately locate the indexing gear consistently for one braking operation after another. This lack of consistency is due in part to the failure of such braking devices to repeatedly accommodate backlash in a predictable manner so as to remove the effects of such backlash on the indexing operation.
Apart from the use of known indexing mechanisms and index tables, it is also conventional to provide gear drive devices in certain nonrelated apparatuses, wherein the gear drives include anti-backlash mechanisms for reducing the effects of backlash between the gears of the drive. Such anti-backlash mechanisms are known to employ spring biasing means for biasing one or more gears into contact with adjacent gears such that backlash is reduced. However, in these conventional devices, if a rotational force greater than the force of the spring biasing means is exerted on the biased gear in a direction opposite to that of the biasing means, movement between the gears occurs and the problems inherent with backlash recur.